Time course and magnitude of movement-related gating of tactile detection in humans. III. Effect of motor tasks

How visuomotor predictability and task demands affect tactile sensitivity on a moving limb during object interaction in a virtual environment

Tactile sensitivity is decreased on a moving limb compared to the same static limb. This tactile suppression likely reflects an interplay between sensorimotor predictions and sensory feedback. Here, we examined how visuomotor predictability influences tactile suppression. Participants were instructed to hit an approaching virtual object, with the object either never rotating, or always rotating, or rotating unpredictably, prompting related movement adjustments. We probed tactile suppression by delivering a vibrotactile stimulus of varying intensities to the moving hand briefly after the object's rotation and asked participants to indicate if they had felt a vibration. We hypothesized that Unpredictable Rotations would require upweighting of somatosensory feedback from the hand and therefore decrease suppression. Instead, we found stronger suppression with unpredictable than Predictable Rotations. This finding persisted even when visual input from the moving hand was removed and participants had to rely solely on somatosensory feedback of their hand. Importantly, we found a correlation between task demand and tactile suppression in both experiments, indicating that task load can amplify tactile suppression, possibly by downweighting task-irrelevant somatosensory feedback signals to allow for successful task performance when visuomotor task demands are high.

Review of electromyography onset detection methods for real-time control of robotic exoskeletons

BACKGROUND

Electromyography (EMG) is a classical technique used to record electrical activity associated with muscle contraction and is widely applied in Biomechanics, Biomedical Engineering, Neuroscience and Rehabilitation Robotics. Determining muscle activation onset timing, which can be used to infer movement intention and trigger prostheses and robotic exoskeletons, is still a big challenge. The main goal of this paper was to perform a review of the state-of-the-art of EMG onset detection methods. Moreover, we compared the performance of the most commonly used methods on experimental EMG data.

METHODS

A total of 156 papers published until March 2022 were included in the review. The papers were analyzed in terms of application domain, pre-processing method and EMG onset detection method. The three most commonly used methods [Single (ST), Double (DT) and Adaptive Threshold (AT)] were applied offline on experimental intramuscular and surface EMG signals obtained during contractions of ankle and knee joint muscles.

RESULTS

Threshold-based methods are still the most commonly used to detect EMG onset. Compared to ST and AT, DT required more processing time and, therefore, increased onset timing detection, when applied on experimental data. The accuracy of these three methods was high (maximum error detection rate of 7.3%), demonstrating their ability to automatically detect the onset of muscle activity. Recently, other studies have tested different methods (especially Machine Learning based) to determine muscle activation onset offline, reporting promising results.

CONCLUSIONS

This study organized and classified the existing EMG onset detection methods to create consensus towards a possible standardized method for EMG onset detection, which would also allow more reproducibility across studies. The three most commonly used methods (ST, DT and AT) proved to be accurate, while ST and AT were faster in terms of EMG onset detection time, especially when applied on intramuscular EMG data. These are important features towards movement intention identification, especially in real-time applications. Machine Learning methods have received increased attention as an alternative to detect muscle activation onset. However, although several methods have shown their capability offline, more research is required to address their full potential towards real-time applications, namely to infer movement intention.

Sensorimotor memories influence movement kinematics but not associated tactile processing

When interacting with objects, we often rely on visual information. However, vision is not always the most reliable sense for determining relevant object properties. For example, when the mass distribution of an object cannot be inferred visually, humans may rely on predictions about the object's dynamics. Such predictions may not only influence motor behavior but also associated processing of movement-related afferent information, leading to reduced tactile sensitivity during movement. We examined whether predictions based on sensorimotor memories influence grasping kinematics and associated tactile processing. Participants lifted an object of unknown mass distribution and reported whether they detected a tactile stimulus on their grasping hand during the lift. In Experiment 1, the mass distribution could change from trial to trial, whereas in Experiment 2, we intermingled longer with shorter parts of constant and variable mass distributions, while also providing implicit or explicit information about the trial structure. In both experiments, participants grasped the object by predictively choosing contact points that would compensate the mass distribution experienced in the previous trial. Tactile suppression during movement, however, was invariant across conditions. These results suggest that predictions based on sensorimotor memories can influence movement kinematics but not associated tactile perception.

Phantom touch illusion, an unexpected phenomenological effect of tactile gating in the absence of tactile stimulation

We report the presence of a tingling sensation perceived during self-touch without physical stimulation. We used immersive virtual reality scenarios in which subjects touched their body using a virtual object. This touch resulted in a tingling sensation corresponding to the location touched on the virtual body. We called it "phantom touch illusion" (PTI). Interestingly, the illusion was also reported when subjects touched invisible (inferred) parts of their limb. We reason that this PTI results from tactile gating process during self-touch if there is no tactile input to supress. The reported PTI when touching invisible body parts indicates that tactile gating is not exclusively based on vision, but rather on multi-sensory, top-down input involving body schema. This supplementary finding shows that representations of one's own body are defined top-down, beyond the available sensory information.

Tactile facilitation during actual and mere expectation of object reception

During reaching and grasping movements tactile processing is typically suppressed. However, during a reception or catching task, the object can still be acquired but without suppressive processes related to movement execution. Rather, tactile information may be facilitated as the object approaches in anticipation of object contact and the utilization of tactile feedback. Therefore, the current study investigated tactile processing during a reception task. Participants sat with their upper limb still as an object travelled to and contacted their fingers. At different points along the object's trajectory and prior to contact, participants were asked to detect tactile stimuli delivered to their index finger. To understand if the expectation of object contact contributed to any modulation in tactile processing, the object stopped prematurely on 20% of trials. Compared to a pre-object movement baseline, relative perceptual thresholds were decreased throughout the object's trajectory, and even when the object stopped prematurely. Further, there was no evidence for modulation when the stimulus was presented shortly before object contact. The former results suggest that tactile processing is facilitated as an object approaches an individual's hand. As well, we purport that the expectation of tactile feedback drives this modulation. Finally, the latter results suggest that peripheral masking may have reduced/abolished any facilitation.

Facilitation of tactile processing during action observation of goal-directed reach and grasp movements

Our perception of sensory events can be altered by action, but less is known about how our perception can be altered by action observation. For example, our ability to detect tactile stimuli is reduced when our limb is moving, and task-relevance and movement speed can alter such tactile detectability. During action observation, however, the relationship between tactile processing and such modulating factors is not known. Thus, the current study sought to explore tactile processing at a task-relevant location during the observation of reaching and grasping movements performed at different speeds. Specifically, participants observed videos of an anonymous model performing movements at a slow (i.e., peak velocity [PV]: 155 mm/second), medium (i.e., PV: 547 mm/s), or fast speed (i.e., PV: 955 mm/s). To assess tactile processing, weak electrical stimuli of different amplitudes were presented to participants' right thumbs when the observed model was at their starting position and prior to any movement, or when the observed model's limb reached its PV. When observing slow movements, normalized perceptual thresholds were significantly lower/ better than for the pre-movement stimulation time. These data suggest that the movement speed can modulate tactile processing, even when observing a movement. Further, these findings provide seminal evidence for tactile facilitation at a task-relevant location during the observation of slow reaching and grasping movements (i.e., speeds associated with tactile exploration).

Sensory suppression and increased neuromodulation during actions disrupt memory encoding of unpredictable self-initiated stimuli

Actions modulate sensory processing by attenuating responses to self- compared to externally generated inputs, which is traditionally attributed to stimulus-specific motor predictions. Yet, suppression has been also found for stimuli merely coinciding with actions, pointing to unspecific processes that may be driven by neuromodulatory systems. Meanwhile, the differential processing for self-generated stimuli raises the possibility of producing effects also on memory for these stimuli; however, evidence remains mixed as to the direction of the effects. Here, we assessed the effects of actions on sensory processing and memory encoding of concomitant, but unpredictable sounds, using a combination of self-generation and memory recognition task concurrently with EEG and pupil recordings. At encoding, subjects performed button presses that half of the time generated a sound (motor-auditory; MA) and listened to passively presented sounds (auditory-only; A). At retrieval, two sounds were presented and participants had to respond which one was present before. We measured memory bias and memory performance by having sequences where either both or only one of the test sounds were presented at encoding, respectively. Results showed worse memory performance - but no differences in memory bias -, attenuated responses, and larger pupil diameter for MA compared to A sounds. Critically, the larger the sensory attenuation and pupil diameter, the worse the memory performance for MA sounds. Nevertheless, sensory attenuation did not correlate with pupil dilation. Collectively, our findings suggest that sensory attenuation and neuromodulatory processes coexist during actions, and both relate to disrupted memory for concurrent, albeit unpredictable sounds.

Tactile suppression stems from specific sensorimotor predictions

Tactile sensations on a moving hand are perceived weaker than when presented on the same but stationary hand. There is an ongoing debate about whether this weaker perception is based on sensorimotor predictions or is due to a blanket reduction in sensitivity. Here, we show greater suppression of sensations matching predicted sensory feedback. This reinforces the idea of precise estimations of future body sensory states suppressing the predicted sensory feedback. Our results shine light on the mechanisms of human sensorimotor control and are relevant for understanding clinical phenomena related to predictive processes.

The ability to sample sensory information with our hands is crucial for smooth and efficient interactions with the world. Despite this important role of touch, tactile sensations on a moving hand are perceived weaker than when presented on the same but stationary hand. This phenomenon of tactile suppression has been explained by predictive mechanisms, such as internal forward models, that estimate future sensory states of the body on the basis of the motor command and suppress the associated predicted sensory feedback. The origins of tactile suppression have sparked a lot of debate, with contemporary accounts claiming that suppression is independent of sensorimotor predictions and is instead due to an unspecific mechanism. Here, we target this debate and provide evidence for specific tactile suppression due to precise sensorimotor predictions. Participants stroked with their finger over textured objects that caused predictable vibrotactile feedback signals on that finger. Shortly before touching the texture, we probed tactile suppression by applying external vibrotactile probes on the moving finger that either matched or mismatched the frequency generated by the stroking movement along the texture. We found stronger suppression of the probes that matched the predicted sensory feedback. These results show that tactile suppression is specifically tuned to the predicted sensory states of a movement.

Predictive attenuation of touch and tactile gating are distinct perceptual phenomena

In recent decades, research on somatosensory perception has led to two important observations. First, self-generated touches that are predicted by voluntary movements become attenuated compared with externally generated touches of the same intensity (attenuation). Second, externally generated touches feel weaker and are more difficult to detect during movement than at rest (gating). At present, researchers often consider gating and attenuation the same suppression process; however, this assumption is unwarranted because, despite more than 40 years of research, no study has combined them in a single paradigm. We quantified how people perceive self-generated and externally generated touches during movement and rest. We show that whereas voluntary movement gates the precision of both self-generated and externally generated touch, the amplitude of self-generated touch is robustly attenuated compared with externally generated touch. Furthermore, attenuation and gating do not interact and are not correlated, and we conclude that they represent distinct perceptual phenomena.

•

We tested the perception of self-generated and external touch during movement and rest

•

The intensity of self-generated touch is reduced during movement and rest (attenuation)

•

The precision of self-generated and external touch is reduced during movement (gating)

•

Attenuation and gating neither interact nor correlate, and are distinct phenomena

Linking Signal Relevancy and Intensity in Predictive Tactile Suppression

Predictable somatosensory feedback leads to a reduction in tactile sensitivity. This phenomenon, called tactile suppression, relies on a mechanism that uses an efference copy of motor commands to help select relevant aspects of incoming sensory signals. We investigated whether tactile suppression is modulated by (a) the task-relevancy of the predicted consequences of movement and (b) the intensity of related somatosensory feedback signals. Participants reached to a target region in the air in front of a screen; visual or tactile feedback indicated the reach was successful. Furthermore, tactile feedback intensity (strong vs. weak) varied across two groups of participants. We measured tactile suppression by comparing detection thresholds for a probing vibration applied to the finger either early or late during reach and at rest. As expected, we found an overall decrease in late-reach suppression, as no touch was involved at the end of the reach. We observed an increase in the degree of tactile suppression when strong tactile feedback was given at the end of the reach, compared to when weak tactile feedback or visual feedback was given. Our results suggest that the extent of tactile suppression can be adapted to different demands of somatosensory processing. Downregulation of this mechanism is invoked only when the consequences of missing a weak movement sequence are severe for the task. The decisive factor for the presence of tactile suppression seems not to be the predicted action effect as such, but the need to detect and process anticipated feedback signals occurring during movement.

Increased upper-limb sensory attenuation with age

The pressure of our own finger on the arm feels differently than the same pressure exerted by an external agent: the latter involves just touch, whereas the former involves a combination of touch and predictive output from the internal model of the body. This internal model predicts the movement of our own finger and hence the intensity of the sensation of the finger press is decreased. A decrease in intensity of the self-produced stimulus is called sensory attenuation. It has been reported that, due to decreased proprioception with age and an increased reliance on the prediction of the internal model, sensory attenuation is increased in older adults. In this study, we used a force-matching paradigm to test if sensory attenuation is also present over the arm and if aging increases sensory attenuation. We demonstrated that, while both young and older adults overestimate a self-produced force, older adults overestimate it even more showing an increased sensory attenuation. In addition, we also found that both younger and older adults self-produce higher forces when activating the homologous muscles of the upper limb. While this is traditionally viewed as evidence for an increased reliance on internal model function in older adults because of decreased proprioception, proprioception appeared unimpaired in our older participants. This begs the question of whether an age-related decrease in proprioception is really responsible for the increased sensory attenuation observed in older people.

Just noticeable differences for elbow joint torque feedback

Joint torque feedback is a new and promising means of kinesthetic feedback imposed by a wearable device. The torque feedback provides the wearer temporal and spatial information during a motion task. Nevertheless, little research has been conducted on quantifying the psychophysical parameters of how well humans can perceive external torques under various joint conditions. This study aims to investigate the just noticeable difference (JND) perceptual ability of the elbow joint to joint torques. The paper focuses on the ability of two primary joint proprioceptors, the Golgi-tendon organ (GTO) and muscle spindle (MS), to detect elbow torques, since touch and pressure sensors were masked. We studied 14 subjects while the arm was isometrically contracted (static condition) and was moving at a constant speed (dynamic condition). In total there were 10 joint conditions investigated, which varied the direction of the arm's movement and the preload direction as well as torque direction. The JND torques under static conditions ranged from 0.097 Nm with no preload to 0.197 Nm with a preload of 1.28 Nm. The maximum dynamic JND torques were 0.799 Nm and 0.428 Nm, when the arm was flexing and extending at 213 degrees per second, respectively.

Overpressure on fingertips prevents state estimation of the pen grip force and movement accuracy

We tested the hypothesis that the inability to move a pen accurately in a graphic task is partly due to a decrease of afferent somatosensory information resulting from overpressure on the tactile receptors of the fingers holding the pen. To disentangle the depressed somatosensory origin from an altered motor command, we compared a condition in which the participant actively produces pressure on the pen (active grip) with a condition in which pressure is passively applied (passive grip, no grip-related motor command). We expected that the response of the somatosensory cortex to electric stimulation of the wrist's tactile nerve (i.e., SEP) would be greater in the natural pen grip (baseline condition) than in the two overpressure conditions (actively or passively induced). Fifteen adults were required to trace a geometrical shape in the three grip conditions. The SEP amplitude was not significantly different between the baseline and both overpressure conditions. However, behavioral results showed that drawing accuracy is impaired when the pressure on the pen is increased (passively or actively). Cortical source analyses revealed that the activity of the superior parietal areas (SPL) increased in both overpressure conditions. Our findings suggest that the SPL is critical for sensorimotor integration, by maintaining an internal representation of pen holding. These cortical changes might witness the impaired updating of the finger-pen interaction force for such drawing actions under visual guidance.

Directional biases in whole hand motion perception revealed by mid-air tactile stimulation

Many emerging technologies are attempting to leverage the tactile domain to convey complex spatiotemporal information translated directly from the visual domain, such as shape and motion. Despite the intuitive appeal of touch for communication, we do not know to what extent the hand can substitute for the retina in this way. Here we ask whether the tactile system can be used to perceive complex whole hand motion stimuli, and whether it exhibits the same kind of established perceptual biases as reported in the visual domain. Using ultrasound stimulation, we were able to project complex moving dot percepts onto the palm in mid-air, over 30 cm above an emitter device. We generated dot kinetogram stimuli involving motion in three different directional axes ('Horizontal', 'Vertical', and 'Oblique') on the ventral surface of the hand. Using Bayesian statistics, we found clear evidence that participants were able to discriminate tactile motion direction. Furthermore, there was a marked directional bias in motion perception: participants were both better and more confident at discriminating motion in the vertical and horizontal axes of the hand, compared to those stimuli moving obliquely. This pattern directly mirrors the perceptional biases that have been robustly reported in the visual field, termed the 'Oblique Effect'. These data demonstrate the existence of biases in motion perception that transcend sensory modality. Furthermore, we extend the Oblique Effect to a whole hand scale, using motion stimuli presented on the broad and relatively low acuity surface of the palm, away from the densely innervated and much studied fingertips. These findings highlight targeted ultrasound stimulation as a versatile method to convey potentially complex spatial and temporal information without the need for a user to wear or touch a device.

Encoding of limb state by single neurons in the cuneate nucleus of awake monkeys

The cuneate nucleus (CN) is among the first sites along the neuraxis where proprioceptive signals can be integrated, transformed, and modulated. The objective of the study was to characterize the proprioceptive representations in CN. To this end, we recorded from single CN neurons in three monkeys during active reaching and passive limb perturbation. We found that many neurons exhibited responses that were tuned approximately sinusoidally to limb movement direction, as has been found for other sensorimotor neurons. The distribution of their preferred directions (PDs) was highly nonuniform and resembled that of muscle spindles within individual muscles, suggesting that CN neurons typically receive inputs from only a single muscle. We also found that the responses of proprioceptive CN neurons tended to be modestly amplified during active reaching movements compared to passive limb perturbations, in contrast to cutaneous CN neurons whose responses were not systematically different in the active and passive conditions. Somatosensory signals thus seem to be subject to a "spotlighting" of relevant sensory information rather than uniform suppression as has been suggested previously.NEW & NOTEWORTHY The cuneate nucleus (CN) is the somatosensory gateway into the brain, and only recently has it been possible to record these signals from an awake animal. We recorded single CN neurons in monkeys. Proprioceptive CN neurons appear to receive input from very few muscles, and their sensitivity to movement changes reliably during reaching relative to passive arm perturbations. Sensitivity is generally increased, but not exclusively so, as though CN "spotlights" critical proprioceptive information during reaching.

Motor action changes pain perception: a sensory attenuation paradigm in the context of pain

ABSTRACT

A large body of evidence indicates how pain affects motor control, yet the way the motor system influences pain perception remains unclear. We present 2 experiments that investigated sensory attenuation of pain implementing a 2-alternative forced choice paradigm. Particularly, healthy participants received painful stimuli on a moving and nonmoving hand during the execution or the preparation of reaching motor actions. At the end of each trial, they indicated on which hand they perceived the stimulus stronger. The point of subjective equality was obtained to measure sensory attenuation. The intensity (experiment 1) and the threat value (experiment 2) of the pain stimuli were manipulated between-subjects to examine their impact on sensory attenuation. Results of experiment 1 (N = 68) revealed that executing a motor action attenuates pain processing in the moving hand. Sensory attenuation during motor preparation alone occurred with stronger stimulus intensities. Sensory attenuation was not affected by the intensity of the pain stimuli. Results of experiment 2 (N = 79) replicated the phenomenon of sensory attenuation of pain during motor action execution. However, sensory attenuation was not affected by the threat value of pain. Together these findings indicate that executing, but not preparing, a motor action affects pain processing in that body part. No significant associations were found between sensory attenuation indices and inhibitory control abilities or pain catastrophizing, vigilance and rumination. These results provide insight into the inhibitory effects of motor actions on pain processing, suggesting that pain perception is a dynamic experience susceptible to individuals' actions in the environment.

The influence of afferent input on somatosensory suppression during grasping

The processing of somatosensory information is hampered on a moving limb. This suppression has been widely attributed to sensorimotor predictions that suppress the associated feedback, though postdictive mechanisms are also involved. Here, we investigated the extent to which suppression on a limb is influenced by backward somatosensory signals, such as afferents associated with forces that this limb applies. Participants grasped and lifted objects of symmetric and asymmetric mass distributions using a precision grip. We probed somatosensory processing at the moment of the grasp by presenting a vibrotactile stimulus either on the thumb or index finger and asked participants to report if they felt this stimulus. Participants applied greater forces with the thumb and index finger for objects loaded to the thumb’s or index finger’s endpoint, respectively. However, suppression was not influenced by the different applied forces. Suppression on the digits remained constant both when grasping heavier objects, and thus applying even greater forces, and when probing suppression on the skin over the muscle that controlled force application. These results support the idea that somatosensory suppression is predictive in nature while backward masking may only play a minor role in somatosensory processing on the moving hand, at least in this context.

Neural Coding of Contact Events in Somatosensory Cortex

Manual interactions with objects require precise and rapid feedback about contact events. These tactile signals are integrated with motor plans throughout the neuraxis to achieve dexterous object manipulation. To better understand the role of somatosensory cortex in interactions with objects, we measured, using chronically implanted arrays of electrodes, the responses of populations of somatosensory neurons to skin indentations designed to simulate the initiation, maintenance, and termination of contact with an object. First, we find that the responses of somatosensory neurons to contact onset and offset dwarf their responses to maintenance of contact. Second, we show that these responses rapidly and reliably encode features of the simulated contact events-their timing, location, and strength-and can account for the animals' performance in an amplitude discrimination task. Third, we demonstrate that the spatiotemporal dynamics of the population response in cortex mirror those of the population response in the nerves. We conclude that the responses of populations of somatosensory neurons are well suited to encode contact transients and are consistent with a role of somatosensory cortex in signaling transitions between task subgoals.

The Orofacial Somatosensory System Is Modulated During Speech Planning and Production

Purpose In our previous studies, we showed that the brain modulates the auditory system, and the modulation starts during speech planning. However, it remained unknown whether the brain uses similar mechanisms to modulate the orofacial somatosensory system. Here, we developed a novel behavioral paradigm to (a) examine whether the somatosensory system is modulated during speech planning and (b) determine the somatosensory modulation's time course during planning and production. Method Participants (N = 20) completed two experiments in which we applied electrical current stimulation to the lower lip to induce somatosensory sensation. In the first experiment, we used a staircase method (one-up, four-down) to determine each participant's perceptual threshold at rest (i.e., the stimulus that the participant detected on 85% of trials). In the second experiment, we estimated each participant's detection ratio of electrical stimuli (with a magnitude equivalent of their perceptual threshold) delivered at various time points before speaking and during a control condition (silent reading). Results We found that the overall detection ratio in the silent reading condition remained unchanged relative to the detection ratio at rest. Approximately 536 ms before speech onset, the detection ratio in the speaking condition was similar to that in the silent reading condition; however, the detection ratio in the speaking condition gradually started to decrease and reached its lowest level at 58 ms before speech onset. Conclusions Overall, we provided compelling behavioral evidence that, as the speech motor system prepares speech movements, it also modulates the orofacial somatosensory system in a temporally specific manner.

The Evaluation of Tactile Parameters and Display Prototype to Support Physiological Monitoring and Multitasking for Anesthesia Providers in the Operating Room

Communicating physiological information via the tactile modality is shown as a promising means to address data overload faced by anesthesia providers. However, it is important to ensure that the tactile parameters which represent information are intuitive. There is currently no consensus on which tactile parameters should be used to present information within anesthesia. The two studies presented here evaluate: (a) a set of 24 tactile cues manipulating intensity, temporal, and spatial tactile parameters in a usability study and (b) a prototype tactile display based on the usability study's findings in a single and dual-task scenario. Findings of the usability study show intensity and temporal were rated most urgent and had the most potential to represent changes in physiological measures. This was confirmed in the follow up study as increases/decreases in intensity were shown to represent increases/decreases in a physiological measure and using different spatial locations to represent physiological measures resulted in greater than 95% response accuracy. Response times and accuracy were not adversely affected while performing a secondary task. The findings contribute to a better understanding of how to map tactile parameters to physiological information and demonstrate the effectiveness of end-user feedback in tactile display design to develop intuitive alerts.

Seeing your own or someone else's hand moving in accordance with your action: The neural interaction of agency and hand identity

Forward models can predict sensory consequences of self-action, which is reflected by less neural processing for actively than passively generated sensory inputs (BOLD suppression effect). However, it remains open whether forward models take the identity of a moving body part into account when predicting the sensory consequences of an action. In the current study, fMRI was used to investigate the neural correlates of active and passive hand movements during which participants saw either an on-line display of their own hand or someone else's hand moving in accordance with their movement. Participants had to detect delays (0-417 ms) between their movement and the displays. Analyses revealed reduced activation in sensory areas and higher delay detection thresholds for active versus passive movements. Furthermore, there was increased activation in the hippocampus, the amygdala, and the middle temporal gyrus when someone else's hand was seen. Most importantly, in posterior parietal (angular gyrus and precuneus), frontal (middle, superior, and medial frontal gyrus), and temporal (middle temporal gyrus) regions, suppression for actively versus passively generated feedback was stronger when participants were viewing their own compared to someone else's hand. Our results suggest that forward models can take hand identity into account when predicting sensory action consequences.

Limited Pre-Speech Auditory Modulation in Individuals Who Stutter: Data and Hypotheses

Purpose We review and interpret our recent series of studies investigating motor-to-auditory influences during speech movement planning in fluent speakers and speakers who stutter. In those studies, we recorded auditory evoked potentials in response to probe tones presented immediately prior to speaking or at the equivalent time in no-speaking control conditions. As a measure of pre-speech auditory modulation (PSAM), we calculated changes in auditory evoked potential amplitude in the speaking conditions relative to the no-speaking conditions. Whereas adults who do not stutter consistently showed PSAM, this phenomenon was greatly reduced or absent in adults who stutter. The same between-group difference was observed in conditions where participants expected to hear their prerecorded speech played back without actively producing it, suggesting that the speakers who stutter use inefficient forward modeling processes rather than inefficient motor command generation processes. Compared with fluent participants, adults who stutter showed both less PSAM and less auditory-motor adaptation when producing speech while exposed to formant-shifted auditory feedback. Across individual participants, however, PSAM and auditory-motor adaptation did not correlate in the typically fluent group, and they were negatively correlated in the stuttering group. Interestingly, speaking with a consistent 100-ms delay added to the auditory feedback signal-normalized PSAM in speakers who stutter, and there no longer was a between-group difference in this condition. Conclusions Combining our own data with human and animal neurophysiological evidence from other laboratories, we interpret the overall findings as suggesting that (a) speech movement planning modulates auditory processing in a manner that may optimize its tuning characteristics for monitoring feedback during speech production and, (b) in conditions with typical auditory feedback, adults who stutter do not appropriately modulate the auditory system prior to speech onset. Lack of modulation of speakers who stutter may lead to maladaptive feedback-driven movement corrections that manifest themselves as repetitive movements or postural fixations.

Reduced multisensory integration of self-initiated stimuli

The processing and perception of stimuli is altered when these stimuli are not passively presented but rather are actively triggered, or "self-initiated", by the participants. For unimodal stimuli, perceptual changes in stimulus timing and intensity have been demonstrated. Initial results have suggested that self-initiation may affect multisensory processing as well. The present study examined the effects of self-initiation on audiovisual integration in the ventriloquism effect (VE), that is, the mislocalization of auditory stimuli toward a spatially displaced visual stimulus. The effects of self-initiation on the VE were investigated with audiovisual stimuli that featured varying degrees of spatial and temporal separation. Stimuli were either triggered by the participants' button press or not, and stimulus onsets were either predictable or not. Arguing from the perspective of Bayesian causal inference models, we hypothesized self-initiation to increase the prior probability of two stimuli being integrated. Contrary to this intuitive assumption, less VE was observed when the stimuli were self-initiated by the participants than when they were externally generated. Since no effects of self-initiation on unimodal processing were observed, these effects must specifically pertain to multisensory processes. Finally, data were fit with a causal inference model, where self-initiation was associated with a reduction of the prior probability to integrate audiovisual stimuli. In conclusion, the presence of a self-initiated motor signal influences audiovisual integration, such that auditory localization is less biased by visual stimuli, which likely depends on top-down signals.

The role of visual processing on tactile suppression

It has been suggested that tactile signals are suppressed on a moving limb to free capacities for processing other relevant sensory signals. In line with this notion, we recently showed that tactile suppression is indeed stronger in the presence of reach-relevant somatosensory signals. Here we examined whether this effect also generalizes to the processing of additional visual signals during reaching. Brief vibrotactile stimuli were presented on the participants’ right index finger either during right-hand reaching to a previously illuminated target LED, or during rest. Participants had to indicate whether they detected the vibrotactile stimulus or not. The target LED remained off (tactile), or was briefly illuminated (tactile & vis) during reaching, providing additional reach-relevant visual information about the target position. If tactile suppression frees capacities for reach-relevant visual information, suppression should be stronger in the tactile & vis compared to the tactile condition. In an additional visual-discrimination condition (tactile & visDis), the target LED flashed once or twice during reaching and participants had to also report the number of flashes. If tactile suppression occurs to free additional capacities for perception-relevant visual signals, tactile suppression should be even stronger in the tactile & visDis compared to the tactile & vis condition. We found that additional visual signals improved reach endpoint accuracy and precision. In all conditions, reaching led to tactile suppression as indicated by higher detection thresholds compared to rest, confirming previous findings. However, tactile suppression was comparable between conditions arguing against the hypothesis that it frees capacities for processing other relevant visual signals.

Comparing intensities and modalities within the sensory attenuation paradigm: Preliminary evidence

It is well-documented that the intensity of a self-generated somatosensory stimulus is perceived to be attenuated in respect to an identical stimulus generated by others. At present, it is not clear whether such a phenomenon, known as somatosensory attenuation, is based not only on feedforward motor signals but also on re-afferences towards the body. To answer this question, in the present pilot investigation on twelve healthy subjects, three types of stimulations (sensory non-nociceptive electrical – ES, nociceptive electrical – NES, and vibrotactile – VTS) and intensities (1 = sensory threshold ∗ 2.5 + 2 mA, 2 = sensory threshold ∗ 2.5 + 3 mA, 3 = sensory threshold ∗ 2.5 + 4 mA for ES and NES; 1 = sensory threshold ∗ 2 Hz, 2 = sensory threshold ∗ 3 Hz, 3 = sensory threshold ∗ 4 Hz for VTS) have been directly compared in a somatosensory attenuation paradigm. The results show that the attenuation effect emerged only with electrical stimuli and that it increased with higher intensities. These pilot findings suggest that, depending on the type and the intensity of stimulation, re-afferences can have a role in somatosensory attenuation. Additionally, it is possible to speculate the effect is present only with electrical stimuli because those stimuli are prospectively judged as potentially dangerous. This, in turn, would optimize planning successful reactions to incoming threatening stimuli.

Reach-relevant somatosensory signals modulate tactile suppression

Tactile stimuli on moving limbs are typically attenuated during reach planning and execution. This phenomenon has been related to internal forward models that predict the sensory consequences of a movement. Tactile suppression is considered to occur due to a match between the actual and predicted sensory consequences of a movement, which might free capacities to process novel or task-relevant sensory signals. Here, we examined whether and how tactile suppression depends on the relevance of somatosensory information for reaching. Participants reached with their left or right index finger to the unseen index finger of their other hand (body target) or an unseen pad on a screen (external target). In the body target condition, somatosensory signals from the static hand were available for localizing the reach target. Vibrotactile stimuli were presented on the moving index finger before or during reaching or in a separate no-movement baseline block, and participants indicated whether they detected a stimulus. As expected, detection thresholds before or during reaching were higher compared with baseline. Tactile suppression was also stronger for reaches to body targets than external targets, as reflected by higher detection thresholds and lower precision of detectability. Moreover, detection thresholds were higher when reaching with the left than with the right hand. Our results suggest that tactile suppression is modulated by position signals from the target limb that are required to reach successfully to the own body. Moreover, limb dominance seems to affect tactile suppression, presumably due to disparate uncertainty of feedback signals from the moving limb.NEW & NOTEWORTHY Tactile suppression on a moving limb has been suggested to release computational resources for processing other relevant sensory events. In the current study, we show that tactile sensitivity on the moving limb decreases more when reaching to body targets than external targets. This indicates that tactile perception can be modulated by allocating processing capacities to movement-relevant somatosensory information at the target location. Our results contribute to understanding tactile processing and predictive mechanisms in the brain.

Speed of reaction to sensory stimulation is enhanced during movement

We report four experiments on the speed of people's reactions to sensory stimulation while throwing and catching a basketball. Thirty participants participated in Experiment 1, split according to basketball expertise: none, intermediate (6years on average), or advanced (20years or more). The participants had to catch a bouncing basketball. The movement triggered a short tactile pulse in a tactor attached to their wrist to which they made a speeded vocal response (RT). The pulse could be presented either at rest, at two time-points during the reaching movement, or when the hand reached forward to catch the ball. The results indicated that participants responded more rapidly to vibrations on the moving hand relative to preparing or catching the ball, with expert athletes responding significantly faster than novices. In a second experiment, participants made a speeded vocal response to an auditory signal. As in Experiment 1, faster auditory RTs were observed when the hand was moving, as compared to the other time-points. In a third study, the participants responded to a pulse delivered at their resting hand at various time-points corresponding to the average timings of stimulation in Experiment 1. The results revealed comparable RTs across the tested time-points. In a final experiment, the participants made a vocal response to a pulse presented at various time-points while they were throwing the basketball. The results indicated faster tactile RTs while the ball was being thrown. These results are discussed with reference to the literature on goal-directed movements and in terms of current theories of attention and sensory suppression.

Facilitation of cutaneous inputs during the planning phase of gait initiation

It has been shown that during the planning of a voluntary movement the transmission of cutaneous afferent inputs to the somatosensory cortex is attenuated shortly before the motor output as well as during movement execution. However, it is not known whether the sensory suppression observed during the planning phase (i.e., before any movement execution) is a systemic phenomenon or whether it is dependent on movement context. For example, movements such as step initiation are controlled based on information received from cutaneous receptors in the feet. Because afferent information emerging from these receptors is critical for movement initiation, we hypothesized that suppression of these inputs may not occur during the planning phase prior to gait initiation. To examine this hypothesis we measured the cortical response to somatosensory stimulation during the planning phase of step initiation and during movement execution. Sensitivity to cutaneous stimulation was assessed by measuring the amplitude of the cortical somatosensory-evoked potential (SEP, over the Cz electrode) following electrical stimulations of the plantar sole of one foot. Two stimulations were provided during the planning phase of a step movement and two stimulations during movement execution. It was found that the P50-N80 SEP was facilitated in the early planning phase (-700 ms before motor execution) compared with when participants remained still (control standing task). This mechanism might contribute to an enhanced perception of cutaneous input leading to a more accurate setting of the forces to be exerted onto the ground to shift the body's weight toward the supporting side prior to foot-off.

Attention and prediction in human audition: a lesson from cognitive psychophysiology

Attention is a hypothetical mechanism in the service of perception that facilitates the processing of relevant information and inhibits the processing of irrelevant information. Prediction is a hypothetical mechanism in the service of perception that considers prior information when interpreting the sensorial input. Although both (attention and prediction) aid perception, they are rarely considered together. Auditory attention typically yields enhanced brain activity, whereas auditory prediction often results in attenuated brain responses. However, when strongly predicted sounds are omitted, brain responses to silence resemble those elicited by sounds. Studies jointly investigating attention and prediction revealed that these different mechanisms may interact, e.g. attention may magnify the processing differences between predicted and unpredicted sounds. Following the predictive coding theory, we suggest that prediction relates to predictions sent down from predictive models housed in higher levels of the processing hierarchy to lower levels and attention refers to gain modulation of the prediction error signal sent up to the higher level. As predictions encode contents and confidence in the sensory data, and as gain can be modulated by the intention of the listener and by the predictability of the input, various possibilities for interactions between attention and prediction can be unfolded. From this perspective, the traditional distinction between bottom-up/exogenous and top-down/endogenous driven attention can be revisited and the classic concepts of attentional gain and attentional trace can be integrated.

Neck proprioception shapes body orientation and perception of motion

This review article deals with some effects of neck muscle proprioception on human balance, gait trajectory, subjective straight-ahead (SSA), and self-motion perception. These effects are easily observed during neck muscle vibration, a strong stimulus for the spindle primary afferent fibers. We first remind the early findings on human balance, gait trajectory, SSA, induced by limb, and neck muscle vibration. Then, more recent findings on self-motion perception of vestibular origin are described. The use of a vestibular asymmetric yaw-rotation stimulus for emphasizing the proprioceptive modulation of motion perception from the neck is mentioned. In addition, an attempt has been made to conjointly discuss the effects of unilateral neck proprioception on motion perception, SSA, and walking trajectory. Neck vibration also induces persistent aftereffects on the SSA and on self-motion perception of vestibular origin. These perceptive effects depend on intensity, duration, side of the conditioning vibratory stimulation, and on muscle status. These effects can be maintained for hours when prolonged high-frequency vibration is superimposed on muscle contraction. Overall, this brief outline emphasizes the contribution of neck muscle inflow to the construction and fine-tuning of perception of body orientation and motion. Furthermore, it indicates that tonic neck-proprioceptive input may induce persistent influences on the subject's mental representation of space. These plastic changes might adapt motion sensitiveness to lasting or permanent head positional or motor changes.

Motor commands induce time compression for tactile stimuli

Saccades cause compression of visual space around the saccadic target, and also a compression of time, both phenomena thought to be related to the problem of maintaining saccadic stability (Morrone et al., 2005; Burr and Morrone, 2011). Interestingly, similar phenomena occur at the time of hand movements, when tactile stimuli are systematically mislocalized in the direction of the movement (Dassonville, 1995; Watanabe et al., 2009). In this study, we measured whether hand movements also cause an alteration of the perceived timing of tactile signals. Human participants compared the temporal separation between two pairs of tactile taps while moving their right hand in response to an auditory cue. The first pair of tactile taps was presented at variable times with respect to movement with a fixed onset asynchrony of 150 ms. Two seconds after test presentation, when the hand was stationary, the second pair of taps was delivered with a variable temporal separation. Tactile stimuli could be delivered to either the right moving or left stationary hand. When the tactile stimuli were presented to the motor effector just before and during movement, their perceived temporal separation was reduced. The time compression was effector-specific, as perceived time was veridical for the left stationary hand. The results indicate that time intervals are compressed around the time of hand movements. As for vision, the mislocalizations of time and space for touch stimuli may be consequences of a mechanism attempting to achieve perceptual stability during tactile exploration of objects, suggesting common strategies within different sensorimotor systems.

Tactile gating in a reaching and grasping task

Abstract A multitude of events bombard our sensory systems at every moment of our lives. Thus, it is important for the sensory cortex to gate unimportant events. Tactile suppression is a well-known phenomenon defined as a reduced ability to detect tactile events on the skin before and during movement. Previous experiments found detection rates decrease just prior to and during finger abduction, and decrease according to the proximity of the moving effector. This study examined how tactile detection changes during a reach to grasp. Fourteen human participants used their right hand to reach and grasp a cylinder. Tactors were attached to the index finger, the fifth digit, and the forearm of both the right and left arm and vibrated at various epochs relative to a "go" tone. Results showed that detection rates at the forearm decreased before movement onset; whereas at the right index finger, right fifth digit and at the left index finger, left fifth digit, and forearm sites did not decrease like in the right forearm. These results indicate that the task affects gating dynamics in a temporally- and contextually dependent manner and implies that feed-forward motor planning processes can modify sensory signals.

The proprioceptive senses: their roles in signaling body shape, body position and movement, and muscle force

This is a review of the proprioceptive senses generated as a result of our own actions. They include the senses of position and movement of our limbs and trunk, the sense of effort, the sense of force, and the sense of heaviness. Receptors involved in proprioception are located in skin, muscles, and joints. Information about limb position and movement is not generated by individual receptors, but by populations of afferents. Afferent signals generated during a movement are processed to code for endpoint position of a limb. The afferent input is referred to a central body map to determine the location of the limbs in space. Experimental phantom limbs, produced by blocking peripheral nerves, have shown that motor areas in the brain are able to generate conscious sensations of limb displacement and movement in the absence of any sensory input. In the normal limb tendon organs and possibly also muscle spindles contribute to the senses of force and heaviness. Exercise can disturb proprioception, and this has implications for musculoskeletal injuries. Proprioceptive senses, particularly of limb position and movement, deteriorate with age and are associated with an increased risk of falls in the elderly. The more recent information available on proprioception has given a better understanding of the mechanisms underlying these senses as well as providing new insight into a range of clinical conditions.

Perceptual and decisional attenuation of tactile perception during the preparation of self- versus externally-generated movements

We investigated tactile perception during the execution of self- versus externally-generated movements. In a first experiment, we established the temporal characteristics of the movements of interest. In a second experiment, participants had to try to detect a short gap in an otherwise continuous vibratory stimulus delivered to their right wrist under conditions of rest, throwing (i.e., self-initiated movement), or catching a basketball (i.e., externally-generated movement). Our hypothesis was that different patterns of tactile sensitivity (d') and response bias (criteria c and c') would be observed as a function of the timing of gap delivery (i.e., during movement preparation or movement execution) and the type of movement (self- or externally-generated). A third experiment investigated tactile perception at rest while participants adopted different hand postures. This experiment also tested the simple preparation of the self-/externally-generated movements versus the observation of these targeted movements as performed by the experimenter. Due to sensory suppression, participants were significantly less sensitive in detecting the gap in tactile stimulation while executing the movement. Preparing to catch the ball only triggered a shift in response bias (i.e., participants were more liberal/conservative when reporting the gap in stimulation), but no change in perceptual sensitivity was observed, as compared to rest. Preparing to make a ball-throwing movement resulted in a significant decrement in tactile sensitivity, as well as a shift in participants' criterion toward their being more conservative, when responding to the presence of the target. Similar decrements were observed for the observation of self-initiated movement preparation, but not for the observation of their externally-generated counterparts. Taken together, these results demonstrate that different forms of attenuation influence tactile perception, depending on the type of movement that is executed: perceptual and decisional attenuation for self-initiated movements, but only decisional attenuation for externally-generated movements. These results suggest that the movement preparation sensorimotor contingencies are already modulated in prefrontal decision-related cortical brain areas.

Somatosensory factors in taste perception: effects of active tasting and solution temperature

Touch and temperature are recognized as important factors in food perception, but much remains to be learned about how they contribute to the perception of flavor. The present paper describes human psychophysical studies that investigated two recently discovered effects of temperature and mechanical stimulation on taste: (1) enhancement of the savory taste of MSG by active tongue and mouth movements, and (2) modulation of the rate of adaptation to sucrose sweetness by temperature. The first study provides evidence that for MSG but not other taste stimuli, movement of the tongue against the palate enhances taste intensity both by increasing spatial summation between opposing gustatory surfaces and by a hypothesized interaction with touch/kinesthesis. The second study shows that the rate of adaptation to sucrose sweetness (but not quinine bitterness) on the tongue tip is strongly influenced by temperature. It is hypothesized that warming slows adaptation to sucrose by increasing the sensitivity of an early stage of taste transduction. Together these results demonstrate that models of flavor perception must include somatosensory stimuli both as components of flavor perception and as modulators of taste.

Attention and suppression affect tactile perception in reach-to-grasp movements

Reaching with the hand is characterized by a decrease in sensitivity to tactile stimuli presented to the moving hand. Here, we investigated whether tactile suppression can be canceled by attentional orienting. In a first experiment, participants performed a dual-task involving a goal-directed movement paired with the speeded detection of a tactile pulse. The pulse was either delivered to the moving or stationary hand, during movement preparation, execution, or the post-movement phase. Furthermore, stimulation was delivered with equal probability to either hand, or with a higher probability to either the moving or resting hand. The results highlighted faster RTs under conditions of higher probability of stimulation delivery to both moving and resting hands, thus indicating an attentional effect. For the motor preparation period, RTs were faster only at the resting hand under conditions where tactile stimulation was more likely to be delivered there. In a second experiment, a non-speeded perceptual task was used as a secondary task and tactile discrimination thresholds were recorded. Tactile stimulation was delivered concomitantly at both index fingers either in the movement preparation period (both before and after the selection of the movement effector had taken place), in the motor execution period, or, in a control condition, in the time-window of motor execution, but the movement of the hand was restrained. In the preparation period, tactile thresholds were comparable for the two timings of stimulation delivery; i.e., before and after the selection of the movement effector had taken place. These results therefore suggest that shortly prior to, and during, the execution of goal-directed movements, a combined facilitatory and inhibitory influence acts on tactile perception.

Threshold for Detection of Incisal Forces Is Increased by Jaw Movement

Current knowledge regarding the sensitivity of the teeth to forces is based on psychophysical experiments that measured touch detection thresholds under static jaw conditions. It is not known whether jaw movements alter the perception of forces applied to the teeth, but, based on limb movement studies, it is hypothesized that the perception of mechanoreceptor outputs will be downwardly modulated by jaw movements. We predicted that, compared with static jaw conditions, rhythmic jaw movements would be associated with significantly higher psychophysical thresholds for the detection of incisally applied forces. In eight participants, mechanical pulses were delivered to an incisor during static jaw holding or during cyclic jaw opening and closing. Analogous to findings in human limbs, the psychophysical salience of periodontal mechanoreceptor feedback was downwardly modulated by physiologically relevant movements; detection thresholds for mechanical pulses applied to a central incisor were significantly higher during jaw-closing movements than during static jaw positioning.

Real-time decreased sensitivity to an audio-visual illusion during goal-directed reaching

In humans, sensory afferences are combined and integrated by the central nervous system (Ernst MO, Bülthoff HH (2004) Trends Cogn. Sci. 8: 162–169) and appear to provide a holistic representation of the environment. Empirical studies have repeatedly shown that vision dominates the other senses, especially for tasks with spatial demands. In contrast, it has also been observed that sound can strongly alter the perception of visual events. For example, when presented with 2 flashes and 1 beep in a very brief period of time, humans often report seeing 1 flash (i.e. fusion illusion, Andersen TS, Tiippana K, Sams M (2004) Brain Res. Cogn. Brain Res. 21: 301–308). However, it is not known how an unfolding movement modulates the contribution of vision to perception. Here, we used the audio-visual illusion to demonstrate that goal-directed movements can alter visual information processing in real-time. Specifically, the fusion illusion was linearly reduced as a function of limb velocity. These results suggest that cue combination and integration can be modulated in real-time by goal-directed behaviors; perhaps through sensory gating (Chapman CE, Beauchamp E (2006) J. Neurophysiol. 96: 1664–1675) and/or altered sensory noise (Ernst MO, Bülthoff HH (2004) Trends Cogn. Sci. 8: 162–169) during limb movements.

Perception of simulated local shapes using active and passive touch

This study reexamined the perceptual equivalence of active and passive touch using a computer-controlled force-feedback device. Nine subjects explored a 6 x 10-cm workspace, with the index finger resting on a mobile flat plate, and experienced simulated Gaussian ridges and troughs (width, 15 mm; amplitude, 0.5 to 4.5 mm). The device simulated shapes by modulating either lateral resistance with no vertical movement or by vertical movement with no lateral forces, as a function of the digit position in the horizontal workspace. The force profiles and displacements recorded during active touch were played back to the stationary finger in the passive condition, ensuring that stimulation conditions were identical. For the passive condition, shapes simulated by vertical displacements of the finger had lower categorization thresholds and higher magnitude estimates compared with those of active touch. In contrast, the results with the lateral force fields showed that with passive touch, subjects recognized that a stimulus was present but were unable to correctly categorize its shape as convex or concave. This result suggests that feedback from the motor command can play an important role in processing sensory inputs during tactile exploration. Finally, subjects were administered a ring-block anesthesia of the digital nerves of the index finger and subsequently retested. Removing skin sensation significantly increased the categorization threshold for the perception of shapes generated by lateral force fields, but not for those generated by displacement fields.